This invention relates to a moistureproof insulating coating composition for packing circuit boards, more particularly to a moistureproof insulating coating composition for packing circuit boards having slight sags of coating composition and being able to form moistureproof insulating film having uniform film thickness by one treatment.
Heretofore, for moistureproof insulation of a packing circuit board wherein a wiring diagram is printed on a laminated sheet of glass-epoxy resin, paper-phenol resin, or the like and microcomputers, resistors, capacitors, and the like parts are provided thereon, there have been used insulating coating compositions having a viscosity of 2-3 poises at 25.degree. C. But since the thickness of a coating film after cured is about 20-50 .mu.m, in order to obtain reliability of moistureproof insulating protection, coating and drying of coating composition is generally repeated for 2-5 times to give a film thickness of 100-150 .mu.m. Therefore, such a process has disadvangtages not only in making the working procedures complicated and processing cost high but also in generating easily bubbles due to the wet-on-wet coating and lowering insulating properties due to inclusion of moisture and the like in the bubbles. On the other hand, in order to thicken a coating film by a one-step coating procedure, there can be used a coating composition having a high viscosity. But when the viscosity is simply increased, sags of the coating composition are taken place so often that there arises another defect in that a constant thickness of coating film cannot be attained. Therefore, there is no coating composition practically suitable for the one-step coating procedure.